The residual effects of thermal ageing at various temperatures on fibres of the aliphatic polyamide Nylon 6,6 have been studied. Both crystal and macro ...
J O U R N A L OF M A T E R I A L S SCIENCE 37 (2002) 2623-2633
Effect of thermal ageing on Nylon 6,6 fibres A. JAIN,K. VIJAYAN* Materials Science Division, National Aerospace Laboratories, Bangalore 560 017, India E-mail: Kavi(S)css.cmmacs.ernetin
The residual effects of thermal ageing at various temperatures on fibres of the aliphatic polyamide Nylon 6,6 have been studied. Both crystal and macro structural characteristics manifest the residual effects. The former category includes changes in intensity, 29 values and half width. The macro changes include introduction of surface damages in the form of holes, material deposits etc. The fibre also undergoes reduction in weight. The structural changes suggest deterioration in the initial tensile characteristics which has been verified experimentally. A direct correlation between the tensile strength and the angular separation of the equatorial reflections has also been observed. © 2002 Kluwer Academic Publishers
1. Introduction Nylon 6,6 is an aliphatic amide made up of poly (hexamethylene adipamide). The structural formula of Nylon 6,6 is shown below. [-(CH2)6 - NH-CO - (CH2)4 - CO-NH—]„ Nylons have an extensive range of applications viz., as apparel, carpets, tyre reinforcement, parachutes and many other industrial applications. Nylon 6,6 is recommended for use upto 100°C [1]. Do exposures to elevated temperatures affect the initial characteristics of the fibre? This paper reports the residual effects of cumulative thermal exposures or thermal ageing on the initial characteristics of commercially available Nylon 6,6 fibres derived by X-ray diffraction methods, scanning electron microscopy, tensile testing and weight analysis. It must be mentioned that to date, data on the residual effects of thermal ageing on Nylon 6,6 fibres are not available. Radusch et a/.'s [2] X-ray analysis on Nylon 6,6 at elevated temperatures has been carried out with an in situ heating arrangement and it therefore differs from the present study which concerns the residual effects. It must also be mentioned that in the past, extensive investigations have been carried out on the thermal ageing behavior of the aromatic polyamides Kevlar, Nomex and Twaron [3-12]. In this context, it was also of interest to find out whether the behaviors of the aliphatic and the aromatic polyamides were comparable.
2. Experimental details The Nylon 6,6 fibres used in this study were obtained from M/s Goodfellow. Thermal ageing was conducted at three different temperatures (T) viz., 175,225 and 245°C respectively. It must be pointed out that the choice of T's above 100°C was deliberate and was intended to enable accelerated data collection. The total
duration of cumulative exposure, tcum(T) varied with T. Table I lists the T and tcum(T) values chosen. Unconstrained bundles of Nylon 6,6 fibres were aged in air using a tubular resistance furnace which can maintain the temperature to an accuracy of ±2°C. Prior to and at various stages of ageing, the fibres were characterized using various techniques viz., X-ray diffraction, weight analysis, scanning electron microscopy and tensile testing. Wide angle X-ray diffraction patterns were recorded in the reflection geometry, using a Philips powder diffractometer with a proportional counter and a graphite monochromator in the diffracted beam. Cu K^ radiation was used. The samples were rotated at a rate of 1/4° per minute and a chart speed of 10 mm per minute was used. In the equatorial X-ray diffraction pattern from Nylon 6,6, the two most intense reflections are (100) and ((010)+ (110)) occurring at 29 values of ~20.4° and 23.4 respectively, for Cu KQ, radiation. The overlapping reflections (1 10) and (210) occurring at 26 value of ~41.3° are comparatively very weak. The recording of the equatorial X-ray diffraction patterns was therefore confined to the 20 range of 14 to 30°. The indices of the reflections mentioned above are based on the triclinic unit cell of dimensions a = 4.9, b = 5.4 and c = 17.2 A, a = 48.5, ft = 11 and y = 63.5° reported by Bunn and Garner [13]. The parameters 26>max values, half width (&>) and integrated intensity (7) characterizing the diffraction profiles were obtained by fitting a Lorentzian2 function. As is well known, the integrated intensities are related to the crystallinity of the diffracting sample [14]. As the present study concerns only relative changes in crystallinity, no attempt was made to estimate the absolute values of crystallinity. However, the parameter k = /t(r)//o, was used to identify the residual crystallinity of thermally aged fibres. In the above expression for k, the parameters /o and /t(!T) correspond to the total integrated intensities of the reflections recorded from fibres prior to and at various stages of heat treatment respectively.
*Author to whom all correspondence should be addressed. 0022-2461
© 2002 Kluwer Academic Publishers
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T A B L E I T and tcum(T) values chosen for analysis Temperature !T) (- C!
rCum (T )< h)
175 225 245
200, 350.450, 800, 1000, 1350, 1500, 1750 1,2,5, 30, 15, 18,20,22 1,3.5,7. 10, 12, 14, 16
The percentage variation in weight accompanying thermal ageing was estimated as (Auj/U'o) x 100, using a Sartorius analytical balance capable of reading down to 0.0001 g. Here, Aw = WQ — u?t, WQ and wt are the weights of samples prior to and after heat treatment respectively. The latter weight was measured within five minutes of removing the sample from the furnace and cooling to ambient temperature. For each of the chosen experimental conditions, at least three sets of samples were examined and the average value of weight loss was worked out. A Leo scanning electron microscope was used to examine the structural features of gold coated fibres. It must be emphasized that care was taken to avoid manually introduced artifacts and deformations. Prior to examination in the microscope, the surface of fibres was not touched or subjected to any type of bending, twisting etc. Tensile strength, modulus and percentage elongation at break of individual filaments were estimated using a Zwick universal testing machine. For each of the experimental conditions, at least 50 filaments were examined and the average values of tensile properties were estimated.
3. Results and discussion 3.1. X-ray diffraction Fig. la-c present the diffraction patterns recorded prior to and at various stages of ageing at 175, 225 and 245 °C respectively. It must be pointed out that Nylon 6,6 has a as well as ft forms [13]. The a form is, however, the commonly occurring phase. Partial inclusion of the ft form in the sample used in the present study has been ruled out by checking for the reflections from the ft form. In particular, the nonoccurrence of the reflections at 26 values of ~12 and 19° confirmed the absence of the ft form in the sample. The dimensions of the triclinic unit cell characterizing the f$ form are a = 4.9, b = 8.0 and c = 17.2 A, a = 90, ft = 77 and y =67° [131.
3.7.7. Intensities The diffraction patterns in Fig. la-c show that the intensities manifest a progressive variation with temperature as well as the duration of cumulative exposures. It is found that at each of the chosen temperatures, at some stage of the prolonged thermal exposures, the intensities of reflections reach values where they are no longer observable above the background i.e., at this stage of thermal exposure, the sample has no crystalline fraction left to diffract. This stage is referred to as the zero crystallinity state. The time needed for the 100% loss in crystallinity is referred to as /IQO- It must be emphasized that the zero crystallinity state is reached as a 2624
result of cumulative exposure to a constant temperature. It is found that at 175, 225 and 275°C « 1750, 22 and 16 h respectively of exposures are needed to reach the zero ciystallinity state. Fig. 2 presents the logarithmic variation of fjoo with T. Values of A: depicting the progressive diminution and an eventual total loss in crystallinity have been marked in each set of the diffraction patterns in Fig. 1. From these values, the time needed for 50% reduction in the initial crystallinity, /50, has been derived and included in Fig. 2. The parameters /so and fioo are indeed user relevant because from an extra/interpolation of the curves in Fig. 2, it is possible to predict the duration of exposure which will cause any deterioration in the initial crystallinity, for a chosen T value. As is well known, crystallinity is an important characteristic related to the properties of fibres. Variation in crystallinity introduced by isothermal ageing of the type conducted in the present study implies corresponding changes in properties which in turn can affect the performance of the fibre during its service life. Fig. 1 shows that prior to reaching the zero crystallinity state, the relative intensities of the two most intense reflections exhibit an interesting behavior. As can be seen in the patterns from the as received fibres i.e., prior to heat treatment, the peak intensity, Ip(QiO)+(iiQ) ^-fpooo)- However, with continuance of thermal exposures, the relative /p values change. At both 225 and 245°C, the difference between the 7p values reaches a high after 5-7 h of thermal exposure. At this stage, / P (oi0)+(ii0) > /p(ioo)- Beyond this stage there is an overall reduction in the integrated intensities leading, eventually, to a state of nearly zero intensity. As is well known the intensity distribution in an X-ray diffraction pattern depends on the atomic and molecular arrangement. The observations on the time dependent variations in relative intensities suggest that the first few thermal exposures of Nylon 6,6 cause changes in the atomic/molecular arrangement which in turn lead to changes in the relative intensities of the equatorial reflections. Subsequent exposures, however, cause progressive degradation and an eventual total loss in the crystallinity of the fibre. 3.1.2. 26 values It must be pointed out that in the patterns shown in Fig. 1, the ((010)+ (110)) reflections occur at 20 less than the value reported by Bunn and Garner [13] viz., shifted towards the low angle side by ~0.5° on the 29 scale. This feature suggests that the ^-dimension characterizing the samples used in the present study is slightly more than what has been reported for Nylon 6,6 [13]. In the crystal structure of Nylon 6,6 the hydrogenbonded layers are stacked along the crystallographic ^-direction. The enhanced &-value thus suggests that the layers are, initially, farther apart than expected. Fig. 3 presents the variation of 28 values with T and ?cum(2") values. It is observed that 29 values for both the reflections increase initially followed by a decrease. The initial increase which suggests a reduction in the corresponding interplanar spacing (d) could be interpreted as an initial annealing type of effect introduced
emperature. 750, 22 and to reach the logarithmic linution and >een marked ig. 1. From iction in the nd included 2ed user relf the curves ition of exn the initial yell known, ; related to tallinity in: conducted changes in brmance of zero cryse two most shavior. As received fik intensity, inuance of change. At i the 7p val1 exposure. 1 this stage 1 intensities 0 intensity, in an X-ray 1 molecular dependent lat the first changes in n turn lead equatorial , cause proloss in the
As received
| AS received
14
18
22
26
14
30
18
22
26
30
WT)(h)
16
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12
10 CO
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> shown in : at 20 less r [13] viz., i.5° on the -dimension :nt study is r Nylon 6,6 hydrogendlographic ggests that pected. kvith T and is for both a decrease, tion in the d be interintroduced
14
18
22
26
30
20 (°) (c)
Figure 1 (a) X-ray diffraction profiles recorded prior to and at various stages of exposure to 175CC. (b) X-ray diffraction profiles recorded prior to and at various stages of exposure to 225°C. (c) X-ray diffraction profiles recorded prior to and at various stages of exposure to 245°C.
by thermal exposures. The subsequent increase in the ^-spacing may be associated with the residual effects of thermal expansion which have occurred at the respective elevated temperatures. The observed shifts in the 20 values of the equatorial reflections recorded from thermally aged Nylon 6,6 fibres are strikingly different from Radusch et • psuedo hexagonal structural transformation. The observed reduction in the J-spacing is also interpreted as partial breaking 2625
3.4
100000
3.3 1000 €3.2
8 psuedo hexagonal structural transformations at elevated temperatures, the transformation is reversible. On returning to ambient temperatures, where the X-ray diffraction patterns were recorded, the samples are back to the triclinic phase albeit with slight shifts in the unit cell dimensions (Fig. 3). A parameter which is determined by the 20 values of the two equatorial reflections is the angular separation A(20) defined as A(20) = 20(0io)-KiiO) - 20(io i n
ie observed sties, weight cumulative n the initial ;n confirmed
Fig. 18a and b present the typical load-extension curves. The former shows the effect of T and in the latter, the role of tcum(T) for a constant T has been, depicted. The progressive changes in the slope and the 2631
I
t
"J
I
jn
10
0.1
100
1000
WT)(h) (a)
7
Acknowledgement The authors thank Dr. T. S. Prahlad, Director, NAL for the support. One of the authors (AJ) thanks the Naltech Pvt. Ltd, Bangalore, India for providing a contract post and the opportunity to work in the project. The authors wish to thank Dr. N. Balasubramanian and Mr. Basavaraj of Eternit Everest, India, for carrying out the tensile tests and Dr. T. A. Bhaskaran and Ms. Kalavati for recording the micrographs. The authors acknowledge the useful suggestions from Dr. R. V. Krishnan, Head, Materials Science Division, NAL.
a fibre exposed
nperature as tensile modc depict the n the tensile elongation at he values of elongation at mperature T ;ed that after strength deany stage of sile strength tiding reducion of tensile es connected riorate faster lie modulus, tas examined yarn used in :s of temper5 the relative fibres aged at jferential en: these higher •ength appear le associated nultaneously ned earlier in i is accompation between mple has al: case of tenelongation at lulative ther.t this stage, it >ther changes 5,6 fibres are ged aramids, >verall nature
include changes in the crystal structural as well as macro structural characteristics. The former includes conspicuous changes in X-ray crystallinity, unit cell dimensions and half widths. Thermal ageing introduces a progressive diminution and an eventual total loss in the crystallinity of the fibre. Macro changes include introduction of surface damages in the form of holes, material deposit and localized longitudinal deterioration. Thermal ageing also causes weight loss. The tensile modulus, strength and percentage elongation at break manifest reductions from the initial values. The results on Nylon 6,6 fibres indicate that the thermally induced changes are not exclusive to the aromatic polyamides. Similarities in the behavior of the aliphatic and the aromatic polyamides have been established.
1000
References
0.1
1000
Figure 19 (a) Variation of Af,r/M0 with tcum(T) and T. (b) Variation of Sf /S0 with r cum (r) and T. (c) Variation of e?/e0 with tmm(T) and T.
of the thermally induced effects thus appears to be common to aromatic as well as aliphatic poly amides.
1. W. J. ROFF and J . R . SCOTT, in "Fibres, Films, Plastics and Rubbers" (Butterworth, England, 1971) p. 216. 2. H. J. R A D U S C H . M . S T O L P a n d R . A N D R O S C H , Polymer 35(1994)3568. 3. R. V. I Y E R and K. VIJ A V A N , in "Polymer Science Recent Advances," Vol. 1, edited by I. S. Bhardwaj (Allied Publishers, New Delhi, India, 1994) p. 362. 4. Idem., Bull. Mater. Sci. 22 (1999) 1013. 5. R. V. I Y E R , Ph.D. Thesis, Bangalore University, Bangalore, India, 1999. 6. A. J A I N a n d K . VIJ A Y A N , J. Mater. Sci., communicated. 1. Idem., ibid, to be communicated. 8. A. J A I N , Ph.D. Thesis, Mangalore University, Mangalore, India, 2001. 9. R. V. l Y E R a n d K . V I J A Y A N , i n "Macromolecules New Frontiers," Vol. 2, edited by K. S. V. Srinivasan (Allied Publishers, New Delhi, India, 1998) p. 847. 10. Idem., Curr. Sci. 75 (1998) 946. 11. Idem., J. Mater. Sci. 35 (2000) 5731. 12. K. VI] AY AN, Metals, Materials and Processes 12(2000)259. 13. C. W. B U N N a n d E . V. GARNER,Proc./toy.S0c.l89A( 1947) 39. 14. L. E. A L E X A N D E R , in "X-ray Diffraction Methods in Polymer Science" (Wiley-Interscience, New York, 1969) ch. 3. 15. M. S H U B H A , H . V. P A R I M A L A andK. VUA\ AN, J. Mater. Sci. Lett. 10(1991) 1377. 16. H. A U E R B A C H , J. Appl. Polym. Sci. 37 (1989) 2213.
4. Conclusions
The residual effects of thermal ageing on the initial characteristics of the aliphatic polyamide Nylon 6,6
Received 25 October 2001 and accepted 30 January 2002
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